LM2596T-ADJ/NOPB vs. LM2596S-3.3/NOPB: A Practical Comparison
Quick Verdict: For applications requiring a variable output voltage to accommodate different system needs or adapt to changing load conditions, the LM2596T-ADJ/NOPB is the clear winner. However, if your design absolutely requires a fixed 3.3V output and you want to minimize BOM cost and PCB complexity, the LM2596S-3.3/NOPB offers a straightforward solution.
Spec Comparison Table
| Spec | LM2596T-ADJ/NOPB | LM2596S-3.3/NOPB | Notes |
|---|---|---|---|
| Function | Step-Down | Step-Down | Identical functionality. |
| Output Configuration | Positive | Positive | Identical. |
| Topology | Buck | Buck | Identical. |
| Output Type | Adjustable | Fixed | This is the critical difference. |
| Number of Outputs | 1 | 1 | Identical. |
| Input Voltage Min | 4.5V | 4.5V | Identical. |
| Input Voltage Max | 40V | 40V | Identical. |
| Output Voltage Min | 1.2V | 3.3V | LM2596T-ADJ offers a wider range. |
| Output Voltage Max | 37V | - | LM2596T-ADJ offers a wider range. |
| Output Current Max | 3A | 3A | Identical current handling. |
| Switching Frequency Typ | 150kHz | 150kHz | Identical. |
| Synchronous Rectifier | No | No | Identical. |
| Operating Temperature Range | -40°C ~ 125°C | -40°C ~ 125°C | Identical. |
| Mounting Type | Through Hole | Surface Mount | Significant difference impacting PCB layout. |
| Package Case | TO-220-5 Formed Leads | TO-263-6, D2PAK (5 Leads + Tab), TO-263BA | Impacts PCB layout and thermal management. |
| Supplier Device Package | TO-220-5 | TO-263 (DDPAK-5) | Impacts PCB layout and thermal management. |
Design Trade-offs
The most obvious trade-off is the adjustable vs. fixed output voltage. The LM2596T-ADJ provides flexibility, but requires external resistors to set the output, adding a small amount of BOM cost and introducing a potential source of error if not carefully selected and calibrated. The LM2596S-3.3 eliminates this, simplifying the design and guaranteeing a 3.3V output, assuming the internal reference is within spec. The adjustable nature of the T-ADJ also allows for trimming to compensate for component tolerances, potentially improving overall system accuracy.
The package difference – TO-220 for the T-ADJ and a surface-mount option (D2PAK, TO-263) for the S-3.3 – has significant implications for PCB layout and thermal management. The TO-220 is easier to hand-solder for prototypes and low-volume production, but the surface-mount packages generally offer better thermal performance, especially when combined with a heatsink. The D2PAK and TO-263 also allow for closer component placement, potentially reducing trace lengths and parasitic inductance. The TO-220’s formed leads can also make it more difficult to achieve good thermal contact with a heatsink.
Given the identical switching frequency and lack of synchronous rectification, efficiency will be comparable under similar operating conditions. Conduction losses will be slightly higher at higher currents due to the internal MOSFET’s RDS(on). However, the surface-mount packages of the S-3.3, if properly heatsinked, could allow for slightly higher power dissipation before thermal shutdown. There’s no readily available efficiency curve data for either part, so empirical testing is recommended for critical applications.
Gate drive requirements are identical, as both parts utilize the same internal circuitry. Layout sensitivity is typical for older buck regulator designs: minimizing trace inductance in the switch loop is crucial for stable operation. The TO-220’s longer leads can exacerbate this issue if not carefully routed.
Use-case Fit
Choose LM2596T-ADJ/NOPB when…
- You need to power multiple voltage rails from a single input voltage.
- The system requires a variable output voltage to accommodate different load conditions or battery voltages.
- You need to trim the output voltage to compensate for component tolerances or achieve a specific system requirement.
- You are prototyping and prefer the ease of through-hole soldering.
- The application requires a wider output voltage range beyond a fixed 3.3V.
- You’re designing a power supply for a microcontroller with a programmable voltage requirement.
Choose LM2596S-3.3/NOPB when…
- You absolutely need a fixed 3.3V output and nothing else.
- You want to minimize BOM cost and PCB complexity.
- You’re designing a high-volume product where a fixed voltage is sufficient.
- You prefer the smaller footprint and better thermal performance of a surface-mount package.
- You’re designing a simple power supply for a device that only requires 3.3V.
- You’re designing a power supply for a 3.3V logic device and want a straightforward solution.
Drop-in Compatibility
Pin compatibility is not guaranteed. While the pinout is similar, the exact pin assignments may differ between the TO-220 and surface-mount packages. Footprint compatibility is also unlikely, requiring a PCB layout change. The TO-220’s formed leads require a different pad layout than the D2PAK or TO-263. Substituting one part for the other requires a thorough review of the schematic and PCB layout.
Alternatives to Consider
- LM2596S-5.0/NOPB: Similar to the S-3.3, but provides a fixed 5.0V output. Useful when a different fixed voltage is needed.
- LM2576: A smaller, lower-power alternative if the 3A current rating isn’s required.
- TPS54331: A more modern, synchronous buck regulator with improved efficiency and features.
Power Electronics Digest
New references, design guides, and component updates — free.